Radiation sensitive silver halide emulsions containing one or a combination of chloride, bromide and iodide ions have been long recognized to be useful in photography. Each halide ion selection is known to impart particular photographic advantages. By a wide margin the most commonly employed photographic emulsions are silver bromide and bromoiodide emulsions. Although known and used for many years for selected photographic applications, the more rapid developability and the ecological advantages of high chloride emulsions have provided an impetus for employing these emulsions over a broader range of photographic applications. As employed herein the term "high chloride emulsion" refers to a silver halide emulsion containing at least 50 mole percent chloride, based on total silver. The most ecologically attractive high chloride emulsions are those that contain very low levels of iodide ion.
During the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power both on an absolute basis and as a function of binder hardening, more rapid developability, increased thermal stability, increased separation of native and spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multi-emulsion layer formats, can be realized by increasing the proportions of selected tabular grain populations in photographic emulsions.
Although varied definitions have been adopted in defining tabular grain emulsions, there is a general consensus that the functionally significant distinguishing feature of tabular grains lies in the large disparity between tabular grain equivalent circular diameter (ECD, the diameter of a circle having an area equal to the projected area of the tabular grain) and tabular grain thickness (t, the dimension of the tabular grain normal to its opposed parallel major faces). Average tabular grain aspect ratio (ECD/t) and tabularity (ECD/t.sup.2, where ECD and t are each measured in .mu.m) are art accepted quantifiers of this disparity. To distinguish tabular grain emulsions from those that contain only incidental tabular grain inclusions it is also the recognized practice of the art to require that a significant percentage (e.g., greater than 30 percent and more typically greater than 50 percent) of total grain projected area be accounted for by tabular grains.
In almost every instance tabular grain emulsions satisfying grain thickness (t), average aspect ratio (ECD/t), average tabularity (ECD/t.sup.2) and projected area aims have been formed by introducing two or more parallel twin planes into octahedral grains during their preparation. Regular octahedral grains are bounded by {111} crystal faces. The predominant feature of tabular grains formed by twinning are opposed parallel {111} major crystal faces. The major crystal faces have a three fold symmetry, typically appearing triangular or hexagonal.
The formation of tabular grain emulsions containing parallel twin planes is most easily accomplished in the preparation of silver bromide emulsions. The art has developed the capability of including photographically useful levels of iodide. The inclusion of high levels of chloride as opposed to bromide, alone or in combination with iodide, has been difficult. Silver chloride differs from silver bromide in exhibiting a much stronger propensity toward the formation of grains with faces lying in {100} crystallographic planes. Unfortunately, twinning of grains bounded by {100} crystal faces does not produce grains having a tabular shape. To produce successfully a high chloride tabular grain emulsion by twinning, conditions must be found that favor both the formation of twin planes and {111} crystal faces. Further, after the emulsion has been formed, care in subsequent handling must be exercised to avoid reversion of the grains to their favored more stable form exhibiting {100} crystal faces.
Wey U.S. Pat. No. 4,399,215 produced the first silver chloride high aspect ratio (ECD/t&gt;8) tabular grain emulsion. The tabular grains were of the twinned type, exhibiting major faces of three fold symmetry lying in {111} crystallographic planes. An ammoniacal double-jet precipitation technique was employed. The thicknesses of the tabular grains were high compared to contemporaneous silver bromide and bromoiodide tabular grain emulsions because the ammonia ripening agent thickened the tabular grains. To achieve ammonia ripening it was also necessary to precipitate the emulsions at a relatively high pH, which is known to produce elevated minimum densities (fog) in high chloride emulsions. Further, to avoid degrading the tabular grain geometries sought both bromide and iodide ions were excluded from the tabular grains early in their formation.
Wey et al U.S. Pat. No. 4,414,306 developed a twinning process for preparing silver chlorobromide emulsions containing up to 40 mole percent chloride based on total silver. This process of preparation has not been successfully extended to high chloride emulsions.
Maskasky U.S. Pat. No. 4,400,463 (hereinafter designated Maskasky I) developed a strategy for preparing a high chloride emulsion containing tabular grains with parallel twin planes and {111} major crystal faces with the significant advantage of tolerating significant internal inclusions of the other halides. The strategy was to use a particularly selected synthetic polymeric peptizer in combination with a grain growth modifier having as its function to promote the formation of {111} crystal faces. Adsorbed aminoazaindenes, preferably adenine, and iodide ions were disclosed to be useful grain growth modifiers.
Maskasky U.S. Pat. No. 4,713,323 (hereinafter designated Maskasky II), significantly advanced the state of the art by preparing high chloride emulsions containing tabular grains with parallel twin planes and {111} major crystal faces using an aminoazaindene growth modifier and a gelatino-peptizer containing up to 30 micromoles per gram of methionine. Since the methionine content of a gelatino-peptizer, if objectionably high, can be readily reduced by treatment with a strong oxidizing agent (or alkylating agent, King et al U.S. Pat. No. 4,942,120), Maskasky II placed within reach of the art high chloride tabular grain emulsions with significant bromide and iodide ion inclusions prepared starting with conventional and universally available peptizers.
Maskasky I and II have stimulated further investigations of grain growth modifiers capable of preparing high chloride emulsions of similar tabular grain content. Tufano et al U.S. Pat. No. 4,804,621 employed di(hydroamino)azines as grain growth modifiers; Takada et al U.S. Pat. No. 4,783,398 employed heterocycles containing a divalent sulfur ring atom; Nishikawa et al U.S. Pat. No. 4,952,491 employed spectral sensitizing dyes and divalent sulfur atom containing heterocycles and acyclic compounds; and Ishiguro et al U.S. Pat. No. 4,983,508 employed organic bis-quaternary amine salts.
Bogg U.S. Pat. No. 4,063,951 reported the first tabular grain emulsions in which the tabular grains had parallel {100} major crystal faces. The tabular grains of Bogg exhibited square or rectangular major faces, thus lacking the three fold symmetry of conventional tabular grain {111} major crystal faces. Bogg employed an ammoniacal ripening process for preparing the tabular grains, thereby encountering the grain thickening and pH disadvantages discussed above in connection with Wey. Bogg conceded the process was feasible for producing individual grain aspect ratios no higher than 7:1. Thus, the average aspect ratio of a tabular grain emulsion so produced would necessarily be substantially less than 7. This is corroborated by Example 3 (the only emulsion described with grain features numerically characterized). The average aspect ratio of the emulsion was 2, with the highest aspect ratio grain (grain A in FIG. 3) being only 4. Bogg states that the emulsions can contain no more than 1 percent iodide and demonstrates only a 99.5% bromide 0.5% iodide emulsion.
Mignot U.S. Pat. No. 4,386,156 represents an improvement over Bogg in that the disadvantages of ammoniacal ripening were avoided in preparing a silver bromide emulsion containing tabular grains with square and rectangular major faces. Mignot specifically requires ripening in the absence of silver halide ripening agents other than bromide ion (e.g., thiocyanate, thioether or ammonia). Mignot relies on excess bromide ion for ripening. Since silver bromide exhibits a solubility approximately two orders of magnitude lower than that of silver chloride, reliance on excess bromide ion for ripening precludes the formation of high chloride tabular grains.
Endo and Okaji, "An Empirical Rule to Modify the Habit of Silver Chloride to form Tabular Grains in an Emulsion", The Journal of Photographic Science, Vol. 36, pp. 182-188, 1988, discloses silver chloride emulsions prepared in the presence of a thiocyanate ripening agent. Emulsion preparations by the procedures disclosed has produced emulsions containing a few tabular grains within a general grain population exhibiting mixed {111} and {100} faces.
Mumaw and Haugh, "Silver Halide Precipitation Coalescence Processes", Journal of Imaging Science, Vol. 30, No. 5, Sept./Oct. 1986, pp. 198-299, is essentially cumulative with Endo and Okaji, with section IV-B being particularly pertinent.
Symposium: Torino 1963, Photographic Science, Edited by C. Semerano and U. Mazzucato, Focal Press, pp. 52-55, discloses the ripening of a cubic grain silver chloride emulsion for several hours at 77.degree. C. During ripening tabular grains emerged and the original cubic grains were depleted by Ostwald ripening. As demonstrated by the comparative Example below, after 3 hours of ripening tabular grains account for only a small fraction of the total grain projected area, and only a small fraction of the tabular grains were less than 0.3 .mu.m in thickness. In further investigations going beyond the actual teachings provided, extended ripening eliminated many of the smaller cubic grains, but also degraded many of the tabular grains to thicker forms.
Japanese published patent application (Kokai) 02/024,643, laid open Jan. 26, 1990, was cited in a Patent Cooperation Treaty search report as being pertinent to the tabular grain structures claimed, but is in Applicant's view unrelated. The claim is directed to a negative working emulsion containing a hydrazide derivative and tabular grains with an equivalent circular diameter of 0.6 to 0.2 .mu.m. Only conventional tabular grain preparations are disclosed and only silver bromide and bromoiodide emulsions are exemplified.
Maskasky U.S. Pat. Nos. 5,185,239 and 5,183,732, (hereinafter designated Maskasky IIIa and IIIb) each disclose a process for preparing a high chloride {111} tabular grain emulsion in which silver ion is introduced into a gelatino-peptizer dispersing medium containing a stoichiometric excess of chloride ions of less than 0.5 molar, a pH of at least 4.6, and a grain growth modifier. In Maskasky IIIa the grain growth modifier is a triaminopyrimidine with mutually independent 4, 5 and 6 ring position substitutes, while in Maskasky IIIb the grain growth modifier is adenine.
Maskasky U.S. Pat. No. 5,178,997, (hereinafter designated Maskasky IV) discloses a process for preparing a high chloride {111} tabular grain emulsion in which silver ion is introduced into a gelatino-peptizer dispersing medium containing a stoichiometric excess of chloride ions of less than 0.5 molar and a grain growth modifier of the formula: ##STR1## where Z.sup.2 is --C(R.sup.2).dbd. or --N.dbd.;
Z.sup.3 is --C(R.sup.3).dbd. or --N.dbd.; PA1 Z.sup.4 is --C(R.sup.4).dbd. or --N.dbd.; PA1 Z.sup.5 is --C(R.sup.5).dbd. or --N.dbd.; PA1 Z.sup.6 is --C(R.sup.6).dbd. or --N.dbd.; PA1 with the proviso that no more than one of Z.sup.4, Z.sup.5 and Z.sup.6 is --N.dbd.; PA1 R.sup.2 is H, NH.sub.2 or CH.sub.3 ; PA1 R.sup.3, R.sup.4 and R.sup.5 are independently selected, R.sup.3 and R.sup.5 being hydrogen, halogen, amino or hydrocarbon and R.sup.4 being hydrogen, halogen or hydrocarbon, each hydrocarbon moiety containing from 1 to 7 carbon atoms; and PA1 R.sup.6 is H or NH.sub.2. PA1 R.sup.8 is H, NH.sub.2 or CH.sub.3 ; and
Maskasky and Chang U.S. Pat. No. 5,178,998, (hereinafter designated Maskasky et al) discloses a process for preparing a high chloride tabular grain emulsion in which silver ion is introduced into a gelatino-peptizer dispersing medium containing a stoichiometric excess of chloride ions of less than 0.5 molar and a grain growth modifier of the formula: ##STR2## where Z.sup.8 is --C(R.sup.8).dbd. or --N.dbd.;
R.sup.1 is hydrogen or a hydrocarbon containing from 1 to 7 carbon atoms.